Tomography scanner with axially discontinuous detector array

ABSTRACT

A tomography scanner has intentionally designed, well defined gaps between detector rings with image reconstruction obtained with the use of conventional tomography data processing. The scanner is particularly advantageous as a small animal PET scanner.

CROSS REFERENCE TO RELATED PATENT APPLICATION

This application claims priority from prior provisional patentapplication Ser. No. 60/504,321, filed Sep. 18, 2003, the entiredisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to tomography scanners and, moreparticularly, to positron emission tomography (PET) scanners designedfor imaging small animals or humans.

2. Brief Discussion of the Related Art

Small animal PET scanners are commonly used in research facilities and,desirably, have high spatial resolution and uniformity and highsensitivity as described in “Molecular Imaging of Small Animals withDedicated PET Tomographs,” Chatziioannou, Arion F., European Journal ofNuclear Medicine, Vol. 29, No. 1, January 2002. Commercial examples ofsuch small animal PET scanners are the Concorde R4 and P4 “microPET”small animal PET scanners described in “Performance Evaluation of theMicroPET R4 PET Scanner for Rodents,” Knoess, Christof; Seigel, Stefan;Smith, Anne; Newport, Danny; Richerzhagen, Norbert; Winkeler, Alexandra;Jacobs, Andreas; Goble, Rhonda N.; Graf, Rudolph; Wienhard, Klaus; andHeiss, Wolf-Dieter, European Journal of Nuclear Medicine and MolecularImaging, Vol. 30, No. 5, May 2003 and “Performance Evaluation of theMicroPET P4: A PET System Dedicated to Animal Imaging,” Tai, Y. C.;Chatziioannou, A.; Seigel, S.; Young, J.; Newport, D., Goble, R. N.;Nutt, R. E.; and Cherry, S. R., Physics in Medicine and Biology, 46(2001) 1845-1862, and the Philips “Mosaic” small animal PET scannerdescribed in “Design Evaluation of A-PET: A High Sensitivity Animal PETCamera,” Surti, S.; Karp, J. S.; Perkins, A. E.; Freifelder, R.; andMuehllehner, G., IEEE Transactions on Nuclear Science, Vol. 50, No. 5,October 2003. These scanners utilize dense arrays of small, individualdetector elements, such as scintillation crystals ultimately viewed byphotomultiplier tubes that encode the location of a scintillation event.The terms “detector elements,” “crystals” and “scintillating materials”are used interchangeably herein; however, it should be understood thatthe term “detector elements” includes any elements capable of detectingany type of radiation. The arrays are cylindrically arranged around asmall diameter circle or polygon to form a mass of scintillatingmaterial that is nearly continuous in both the axial and circumferentialdirections. “Continuous” in this sense means that the individualscintillation crystals are as close together as possible such that anyspace between crystals is minimal and small compared to the crystalwidth and that the crystal positioning is replicated along the entireaxial length of the scanning volume without appreciable or well definedgaps between rings of scintillation crystals. “Cylindrical detectorarray” as used herein means any geometric arrangement in whichscintillation crystals or other detector elements circumferentiallysurround an imaging volume, e.g. in a circle, a polygon, an oval or thelike, and have some axial extent. Prior art instrumentation for 3D PETimaging has focused on creating continuous axial and circumferentialarrays of scintillator or other materials able to detect positronannihilation radiation emanating from a stationary imaging target orbody, e.g. a small laboratory animal or a human. From these detectedevents, transverse section images of the radioactivity distribution fromthe body can be reconstructed that span the axial field of view of thedevice. The perceived need for continuity in detector arrays has beensufficiently compelling that prior art scanners have been specificallydesigned to avoid axially discontinuous arrays and have attempted toexploit novel array assembly methods, primarily optical/mechanical, thatallow fabrication of continuous arrays of scintillation crystals, e.g.use of light guide coupling between crystal arrays and photodetectors,that allow close packing of crystals in both the axial andcircumferential directions.

Dedicated PET scanners now on the market for either human or animalimaging targets use continuous cylindrical arrays of small scintillationcrystals to define the imaging volume of the scanner. Discontinuousdetector arrays e.g. paired, opposed flat panel detectors in timecoincidence, are either mechanically rotated around the imaging targetor the imaging target is rotated between fixed detectors to achieve thesame result. See “A Rotating PET Scanner Using BGO Block Detectors:Design, Performance and Applications,” Townsend, David W.; Wensveen,Martin; Byars, Larry G.; Geissbuhler, Antoine; Tochon-Danguy, Henri J.;Christin, Anne; Defrise, Michael; Bailey, Dale L.; Grootoonk, Sylke;Donath, Alfred; and Nutt, Ronald, Journal of Nuclear Medicine, 1993;34:1367-1376, “Design and Characterization of IndyPET-II: A HighResolution, High Sensitivity Dedicated Research Scanner,” Rouze, Ned C.and Hutchins, Gary D., IEEE Transactions on Nuclear Science, Vol. 50,No. 5, October 2003, and “ECAT ART—A Continuously Rotating PET Camera:Performance Characteristics, Initial Clinical Studies, and InstallationConsiderations in a Nuclear Medicine Department,” Bailey, Dale L.;Young, Helen; Bloomfield, Peter M.; Meikle, Steven R.; Glass, Daphne;Meyers, Melvyn J.; Spinks, Terence J.; Watson, Charles C.; Luk, Paul;Peters, A. Michael; and Jones, Terry, European Journal of NuclearMedicine, Vol. 24, No. 1, January 1997. The design of such scanners iscommonly driven by the perceived need to create continuous, or virtuallycontinuous, crystal arrays in the sense defined previously. For example,some of such scanners use individual light guides to connect crystals inan array to a phototube to eliminate the effect of “dead space” at theedges of phototubes. In other scanners, a continuous annulus of glassserves as a light guide to connect the cylindrical array of closelyspaced small crystals to a bank of phototubes. In other scanners, suchas the scanner disclosed in U.S. Pat. No. 6,288,399 to Andreaco et al, alarge, axially continuous polygonal crystal array is created bycentering, and packaging together, many small crystal arrays on clustersof four phototubes, a geometry that allows large arrays to be made byreplication of this pattern as described in “The ECAT HRRT: Performanceand First Clinical Application of the New High Resolution ResearchTomograph,” Weinhard, K.; Schmand, M.; Casey, M. E.; Baker, K.; Bao, J.;Eriksson, L.; Jones, W. F.; Knoess, C.; Lenox, M.; Lercher, M.; Luk, P.;Michel, C.; Reed, J. H.; Richerzhagen, N.; Treffert, J.; Vollmar, S.;Young, J. W.; Heiss, W. D.; and Nutt, R., IEEE Transactions on NuclearScience, Vol. 49, No. 1, February 2002. In each of these cases,technical innovations of one kind or the other are applied to allowsmall scintillation crystals to be packed closely together and to createdetector arrays that are essentially continuous in both thecircumferential and axial directions.

There are two primary reasons why the use of continuous arrays is deemedimportant. First, the idea that continuous arrays will intercept thelargest fraction of annihilation radiation emanating from the targetsubject and, hence, will yield the maximum sensitivity for a particularring diameter and axial length. Second, “classical” information theoryhas been thought to require continuous, regular and dense sampling of animaging volume if imaging performance is to be as good as the systemgeometry permits. It has been generally believed that imagesreconstructed without dense and uniform sampling, i.e. without acontinuous axial and circumferential distribution of scintillatingmaterial, would be of inferior quality, would contain artifacts or both.Degradation has been expected to increase if there were actual gaps inthe detector array in either the circumferential or axial directions. Inparticular, while the effect on image quality of small circumferentialgaps in detector arrays has been studied in some detail, see“Statistical Image Reconstruction in PET with Compensation for MissingData,” Kinahan, P. E.; Fessler, J. A.; and Karp, J. S., IEEETransactions on Nuclear Science, Vol. 44, No. 4, August 1997,“Correction Methods for Missing Data in Sinograms of the HRRT PETScanner,” de Jong, Hugo W. A. M.; Boellaard, Ronald; Knoess, Christof;Lenox, Mark; Michel, Christiaan; Casey, Michael; and Lammertsma, AdriaanA., IEEE Transactions on Nuclear Science, Vol. 50, No. 5, October 2003,and “A Study of Image Errors Due to Detector Gaps Using OS-EMReconstructions,” Yu, D.-C. and Chang, W., IEEE 1998, the literaturecontains little information about changes in image quality if a detectorarray possesses axial gaps, see “Design Optimization of the PMT-ClearPETPrototypes Based on Simulation Studies with GEANT3,” Heinrichs, U.;Pietrzyk, U.; and Ziemons, K., IEEE Transactions on Nuclear Science,Vol. 50, No. 5, October 2003.

While a continuous cylindrical array of scintillation crystalssurrounding an imaging target is an effective way to interceptannihilation radiation from an imaging target, prior art methods possesssignificant practical disadvantages. For example, the need to connectindividual or small groups of scintillation crystals to a phototube witha light guide adds complexity to the manufacturing process. Moreimportantly, there is a demonstrable loss of scintillation light whenthe light passes into and through a light guide, thus, potentiallyreducing imaging performance. A similar loss occurs when a bulk lightguide is used for the same purpose. That is, it has been believed thatthe “dead” regions at the edges of most photonic devices cannot betolerated.

A number of three-dimensional (3D) image reconstruction methods havebeen proposed in recent years including the Fourier re-binning method(FORE) combined with some form of 2D image reconstruction, e.g. filteredbackprojection (FBP) and the 3D re-projection method (3DRP), asdescribed in “Exact and Approximate Rebinning Algorithms for 3-D PETData,” Defrise, Michael; Kinahan, P. E.; Townsend, D. W.; Michel, C.;Sibomana, M.; and Newport, D. F., IEEE Transactions on Medical Imaging,Vol. 16, No. 2, April 1997 and “Performance of the Fourier RebinningAlgorithm for PET with Large Acceptance Angles,” Matej, Samuel; Karp,Joel S.; Lewitt, Robert M.; and Becher, Amir, Phys. Med. Biol. 43 (1998)787-795, which are incorporated herein by reference. Iterative,statistical methods, such as 3D ordered subset expectation maximization(3D OSEM), as described in “Accelerated Image Reconstruction UsingOrdered Subsets of Projection Data,” Hudson, H. Malcolm and Larkin,Richard S., IEEE Transactions on Medical Imaging, Vol. 13, No. 4,December 1994, which is incorporated herein by reference and 3D maximuma posteriori image reconstruction (3D MAP) as described in “HighResolution 3D Bayesian Image Reconstruction Using the MicroPET SmallAnimal Scanner,” Qi, Jinyi; Leahy, Richard M.; Cherry, Simon R.;Chatziioannou, Arion; and Farquhart, Thomas H., Phys. Med. Biol., 43,(1998) 1001-1013, both with system modeling, have also been introduced.A number of other algorithms that exploit the expectationmaximization-maximum likelihood (EM-ML) approach with system modelinghave also been studied as described in “Fast EM-Like Methods for Maximum“A Posteriori” Estimates in Emission Tomography,” De Pierro, Alvaro R.and Yamagishi, Michel Eduardo Beleza, IEEE Transactions on MedicalImaging, Vol. 30, No. 4, April 2001. Each of these methods allows the 3Dinformation potentially available in cylindrical PET scanners withoutcollimators to be reconstructed into 2D slices that fully exploit theincreased sensitivity associated with 3D data collections compared topurely 2D collections. To date, these methods have been used only forimage reconstruction from scanners having continuous cylindrical arraysof scintillation crystals.

SUMMARY OF THE INVENTION

The present invention avoids the need for specially designed arrayassemblies having axially continuous detector arrays by adaptingexisting image reconstruction methods to the presence of axial gaps in adetector array, by mechanical movement of the imaging target relative toan axially discontinuous detector array such that lines-of-response fromparts of the object that might otherwise always lie in a gap aretranslated into locations where a detector array is continuous, and byarranging the detector modules in the detector array such that they aretilted with respect to one another in the axial direction.

In one aspect, the present invention permits the construction of atomography scanner with spaced detector rings allowing direct couplingof scintillators with photon detectors, particularly position-sensitivephotomultiplier tubes, and uses conventional image reconstructionmethods with tomography scanners having axially discontinuous arrays ofdetector rings/scintillators. The effect of gaps in the detector arrayscan be further minimized, if desired, by appropriate movement of theimaging target during imaging or by geometric arrangement of thedetector elements in the cylindrical detector array. In one mode thetomography scanner of the present invention compensates for missing dataintroduced by discontinuities in detector arrays of tomography scannersby using three dimensional (3D) re-binning and/or reconstruction methodsas discussed above.

A tomography scanner according to the present invention includes aplurality of axially aligned detector rings forming a cylindricaldetector array that surrounds the imaging target. This cylindrical arraycontains one or more well defined gaps between the detector rings, whichproduce signals corresponding to points of interaction in threedimensions of radiation from the body within the detector array andmeans responsive to the signals to produce image reconstructions of someaspect of the imaging target throughout the axial field-of-viewincluding the well defined gap.

In accordance with the present invention, a tomography scanner includesa plurality of axially aligned detector rings forming a cylindricaldetector array around a body to be imaged with well defined gaps betweenthe detector rings, the detector rings producing signals in response toradiation emanating from the body in three dimensions and meansresponsive to the signals to produce tomographic image reconstructionthroughout the axial field-of-view including the well defined gaps.

Also, in accordance with the present invention, a positron emissiontomography scanner includes a pair of detector rings forming acylindrical detector array around a body to be imaged with a welldefined gap between the detector rings, a plurality of detector modulescarried by the detector rings producing signals corresponding to thethree dimensional points of interaction in each of a pair of thedetector modules in response to detection of time coincident photonsfrom positron annihilations in a body containing a positron emittingcompound and means responsive to the signals to produce tomographicimage reconstruction of the spatial distribution of the amount ofpositron emitting compound in the body along the axial field-of-view ofthe cylindrical detector array.

In a further aspect, a small animal positron emission tomography scanneraccording to the present invention includes a plurality of spaceddetector rings forming a cylindrical detector array for receiving theanimal with a well defined gap between at least two of the detectorrings which produce signals corresponding to positron annihilationradiation from the animal and means responsive to the signals to produceimage reconstruction of the distribution of a positron emittingradiopharmaceutical in the animal.

The tomography scanner of the present invention compensates for missingaxial data by using 3D iterative, statistical, reconstruction methodsthat do not specifically require complete and uniform spatial sampling,e.g. 3D OSEM, 3D MAP algorithms, and the like as discussed above, andthat incorporate a model of the physics and geometry of the radiationdetection/emission process during image reconstruction.

Some of the advantages of the present invention over the prior art arethat the tomography scanner of the present invention can use lessdetector materials to span the same axial length, is less expensive andeasier to manufacture, permits direct coupling of scintillators tophotomultiplier tubes with substantially less light loss as occurs withlight guides and uses conventional techniques or methods forreconstructing useful images along the full axial length of the scannerincluding the gap regions.

The above and still further features and advantages of the presentinvention will become apparent from the following description ofpreferred embodiments of the invention, particularly when taken inconjunction with the accompanying drawings, wherein like referencenumerals are used to designate like or similar components thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrammatic front and side views, respectively, ofa tomography scanner according to the present invention.

1C is a broken perspective of a detector ring including modules carryingdetector elements.

FIG. 2 is a diagrammatic side view taken along line 2-2 of FIG. 1excluding the body O.

FIG. 3 is a diagrammatic side view taken along line 2-2 of FIG. 1 withthe body O undergoing reciprocating movement.

FIG. 4 is a diagrammatic side view of a modification of a tomographyscanner according to the present invention where crystal arrays aretilted with respect to one another.

FIGS. 5A and 5B are 3D FORE/2D filtered backprojection images of pointsources distributed in a coronal plane containing a ring diameter andimaged with a simulated small animal PET scanner with continuousdetector rings and with a gap between detector rings, respectively.

FIGS. 6A and 6B are 3D FORE/2D filtered backprojection images of acoronal plane through a simulated Defrise phantom containing the axis ofthe scanner and imaged with a simulated small animal PET scanner withcontinuous detector rings and with a gap between detector rings,respectively.

FIG. 6C is an image from a scanner with a gap between detector ringswith data reconstructed with a 3D OSEM algorithm tailored to the gapgeometry.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Tomography and tomographic images refer to images that together portrayin three dimensions some property of an object being imaged. Commonly,such images may be in the form of a sequence of consecutive twodimensional transverse sections closely spaced along the axis of atomography scanner to span the entire axial field-of-view of the scannerand the object therein. A “property” portray in such images can be, butit not limited to, the spatial distribution and frequency of occurrenceof positron annihilation sites in the object, and a “property” may alsorefer to the distribution of attenuation coefficients, the location andamount of a light emitting compound distributed within the object, theamount and location of contrast material introduced into the object, andother such processes and phenomenon.

The term “detector ring” as used herein means an annular structuresurrounding an imaging target or body formed of detector material thatis responsive to incident radiation, such as x-rays, gamma rays, photonpairs from positron annihilation and the like. A detector ring may beformed of various materials and may be assembled in various geometries.Detector rings may be formed of scintillation material, e.g. lutetiumoxyorthosilicate, or may be formed of solid state radiation detectionmaterial, e.g. intrinsic germanium. Detector rings can include materialthat is continuous in the circumferential direction, e.g. an annulus ofscintillation material or solid state detector material, or can includeindependent segments of such materials tiled around the circumference ofa detector ring to form a polygonal, rather than circular, ringgeometry. Such discrete detector segments are referred to herein as“detector modules” to distinguish such segments from continuous rings ofmaterial. Detector modules may be further subdivided into smaller partsreferred to herein as “detector elements”. Commonly, many small detectorelements would be packed together to form a detector module. A detectormodule would be coupled to a device sensitive to emissions from thematerial when struck by incident radiation, and signals from the devicewould serve to identify the location of the point of interaction of theradiation in the detector module ultimately necessary for creation oftomographic images. Solid state or continuous detector modules alsocontain detector elements having points of axial resolution with thespacing therebetween referred to as pitch.

In the case of scintillation detector elements, a number of readoutdevices are available for receiving the signals, including avalanchephotodiodes (APDs) that may be coupled to individual detector elementsin a detector module. Position-sensitive APDs can be coupled to adetector module made up of closely packed arrays of individual,optically isolated detector elements, and arrays of detector elementscan be coupled to position-sensitive photomultiplier tubes. Each ofthese “readout” methods serves the purpose of producing electricalsignals in response to the signals generated in the detector material byincident radiation. The electrical signals encode the position of theinteraction within the detector module in either two or three dimensions(three if the detector module is capable of generating signals that alsodepend on the radial depth of penetration of the radiation into themodule before interaction, i.e. a depth-of-interaction detector module).

In a preferred embodiment, each detector ring is formed of modulescontaining arrays of depth-of-interaction (DOI) capable phoswichscintillator detector elements, each formed of two or more scintillatorswith differing light decay times optically connected to one anotherend-on as described in the above mentioned U.S. Pat. No. 6,288,399 toAndreaco et al, the above mentioned Weinhard et al article and in“Effect of Depth of Interaction Decoding on Resolution in PET: ASimulation Study,” Astakhov, V.; Gumplinger, P.; Moisan, C.; Ruth, T.J.; and Sossi, V., IEEE Transactions on Nuclear Science, Vol. 50, No. 5,October 2003; U.S. Pat. No. 4,843,245 to Lecomte, UK Patent No. GB 2 378112 to Lecoq, and U.S. Pat. No. 6,362,479 B1 to Andreaco et al.Depth-of-interaction information can also be obtained by other meansincluding measurement of the differential output of light from each endof a scintillation crystal of a single type or by the means described inUK Patent No. GB 2 198 620 to Yamashita et al, U.S. Pat. No. 4,831,263to Yamashita, U.S. Pat. No. 6,087,663 to Moisan et al, U.S. Pat. No.5,349,191 to Rogers and U.S. Pat. No. 5,122,667 to Thompson.Depth-of-interaction detectors are preferred because the number oflines-of-response penetrating the imaging volume is increasedsubstantially compared to a non-DOI system with the same general ringand detector geometry and helps reduce the DOI parallax effect in theaxial direction, as well as the transaxial direction. The presentinvention will be described hereinafter relative to a small animal PETscanner; however, it is understood that the concept of the presentinvention can be used with any tomographic scanner acquiring data inthree dimensions, can be used for human or animal imaging and canutilize a variety of ring geometries, detector materials and readoutmethods.

As shown in FIGS. 1A, 1B, 1C and 2, a PET scanner 10 according to thepresent invention includes a cylindrical detector array 11 formed of twoor more detector rings 12 of detector modules 13, the detector ringshaving a diameter D surrounding body O (small animal) forming an imagingtarget or object having a positron annihilation site P and having anangle of obliquitye. The detector modules 13 are made of a matrix(13×13) of detector elements 15. The detector rings 12 are spaced fromeach other by a well defined axial gap G which may or may not beconstant from ring to ring. Dashed lines 14 indicate possiblelines-of-flight of annihilation photons, and arrowheads 16 indicatepoints of interaction within the detector rings. The axial direction isindicated at A. In the preferred embodiment, the detector rings arestationary and the body O or imaging target is also stationary.

Data collection is supplied to a processor means 17 for imagereconstruction. The preferred methods of image reconstruction for PETscanners according to the present invention are iterative statisticalmethods, such as 3D OSEM and 3D MAP, in which a model of the physicaland geometrical response of the imaging system can be included in theimage reconstruction process to correct for deleterious physical and/orgeometrical effects including gaps in the detector array. Thesereconstruction methods could also be applied to the variations describedbelow.

The body O, such as a small animal, can, during imaging, be axiallytranslated within the scanner as shown in FIG. 3, or the scanner can betranslated with respect to the animal, as a means of reducing axialimage degradation caused by gaps or spaces between detector rings inaccordance with the present invention.

Another modification of the PET scanner according to the presentinvention is shown in FIG. 4 wherein detector modules within twodetector rings are tilted or angled toward one another to change theshape of the gap. In this modification, the ring diameter varies alongthe axial direction of the scanner and is not constant. In addition,tilting the detector modules opens small transaxial gaps (not shown inthe drawings) between detector modules, the magnitude of which alsovaries with axial position. Additional detector rings can be added bytiling the perimeter of an axially oriented polygon or an ellipsecentered on the geometric center of the scanner.

It has been observed that a prior art PET scanner with no gaps betweendetector rings will exhibit certain inherent transverse and axialimaging properties when image data are reconstructed with conventional3D methods. In accordance with the present invention, the gap has adimension where axial performance is acceptable compared to otherdistortions introduced by the geometry and/or physical performance of anormal scanner. The term “well defined gap” as used herein refers tointentionally designed gaps between detector rings, as opposed tospacing occurring from designs intended to produce axially continuousdetector rings. The difference between a well defined gap and the verysmall inadvertent spaces that occur in prior art scanners can bedistinguished in the following way. Detector modules formed of arrays ofdetector elements can be characterized in part by their “pitch”, thecenter-to-center spacing between adjacent detector elements. In priorart scanners, the intention has been to span the entire axialfield-of-view of the device with detector elements of constant pitch andto minimize any space between elements such as might be needed tooptically isolate one detector element from adjacent detector elements.If pitch is the sum of the width W of a detector element and T, thethickness of any material between elements, the pitch is P=W+T. In theprior art, the intent has been to make the pitch P as close to W aspossible by minimizing the thickness of intervening material. That is,the width of the spaces is small compared to the axial pitch of thedetector elements. In contrast, gaps according to the present inventionare purposefully chosen such that the gap width between detector ringsis as great as or greater than, the pitch of the detector elements inthe detector modules that form the detector rings.

The gaps G preferably have an axial dimension two to four times greaterthan the axial dimension or pitch of a detector element.

The point and distributed source responses of a Monte Carlo-simulatedcylindrical PET scanner with two detector rings of detector moduleswithout a gap and with a well defined gap equal to four times the pitchof the detector elements in each detector module are shown in FIGS. 5A,6A and 5B, 6B, respectively. In FIGS. 5A and 5B, the simulated data werereconstructed by applying the FORE algorithm followed by 2D filteredback projection (FBP). Images in the gap region are obtained simply byapplying the FORE algorithm to the acquired 3D image data to create the“virtual” sinograms that would have been obtained had the gap regionactually been filled with “real” detector rings and then applying any ofseveral 2D reconstruction methods to these sinograms to obtaintransverse section images of the object. No other computational methodswere used. The FORE+FBP method is an extreme test of the ability of analgorithm to compensate for missing axial data. If such methods yieldmodest reductions in axial imaging performance, iterative statisticalmethods that incorporate accurate models of system geometry and othercharacteristic of the radiation detection process, exhibit even less, orno, reduction in axial imaging performance. With these latter methodsthe activity distribution in the object within the gap region isestimated using lines-of-response that cross the gap region. Inaccordance with the present invention, once the gap distribution isknown or estimated, transverse section images through the distributioncan be created as if there had been actual detector rings that span thegap region.

FIGS. 5A and 5B show point sources spread over a sagittal plane thatcontains the central axis of simulated scanners with no axial gapbetween detector rings and with an axial gap between detector ringsequal to four times the pitch (or axial dimension) of the detectorelements in each module of the detector rings, respectively. The spacingbetween sources is 5 mm in the horizontal and vertical directions. Inthese images, the axial direction is vertical and the radial directionis left and right. As can be seen, the apparent size of the pointsources generally increases as one moves to the left and right away fromthe central axis. This variation is due entirely to the radial parallaxeffect that occurs in all ring-type PET scanners and is present with, orwithout, a gap. There is only a slight apparent difference between therow of points in the 6 mm-wide gap region (middle row of points) of thetwo images, and the overall axial variation is very similar between theimages. The measured transaxial FWHM values in the gap region are nearlyidentical to those in the gap-free image while the axial widths of thespots differ only slightly.

FIGS. 6A and 6B illustrate axial effects revealed by the Defrise phantomwith no axial detector ring gap and an axial gap between detector ringsequal to four times the pitch of the detector elements in each ring,respectively. The phantom is formed of parallel, coaxial cylinders 4.3mm thick, every other one of which filled with a pure positron-emitterand the intervening solid cylinders filled with nothing. The phantomdetects defects that might exist in axial imaging performance and is themost commonly used phantom to reveal axial imaging flaws. Images shownin FIGS. 6A and 6B were reconstructed with the FORE+FBP method. Asnoted, FIG. 6A has no gap and FIG. 6B has an axial gap G equal to fourtimes the detector element pitch in each detector ring. The images ofthe Defrise phantom show a sagittal plane that contains the central axisof the scanner. It would be expected that any distortions due to the“four pitch” gap would be most evident in the central plane (middle row)and would diminish as one moves away from the center along the axis ofthe scanner. In a perfect system, all of the horizontal bands would bethe same width (in the vertical direction). FIGS. 6A and 6B show thatsome degradation occurs in the central plane within the gap region aspredicted by “classical theory”. The central band in the gap image issomewhat wider and fainter than the central band in the no gap imagewhile, away from the central gap in the axial direction, the images arenearly identical. The maximum amplitude of the band at the center of thefield of view in the four pitch-wide gap image is approximately 20% lessthan the same band in the no gap image. A maximum degradation in axialresponse of this magnitude is entirely acceptable in practice given thepractical advantages of being able to use discontinuous detector arrays.Similar experiments using the preferred 3D OSEM algorithm with systemmodel to reconstruct images from a dual ring phoswich-based scanner witha four pitch gap between rings is shown in FIG. 6C and reveals nosignificant degradation in axial resolution (vertical thickness ofslabs). There is no measurable degradation in spatial resolution in thetransverse plane anywhere along the axial field-of-view, outside orinside the gap region, with any of these methods, including the 3DFORE/2D filtered backprojection method, i.e. radial and tangentialspatial resolution in the transverse plane at given radius do not varywith axial position for any of these reconstruction methods.

The overall performance of a tomography scanner with a small axial gapaccording to the present invention, aside from a modest degradation inaxial response in the gap region with the most sensitive method, is notsignificantly different from a tomography scanner without a gap. TheFORE method followed by 2D filtered backprojection were the onlycomputational methods needed to achieve this result, these algorithmsbeing proposed in the past only for use with continuous detector arraysand considered to absolutely require continuous detector arrays. All ofthe 3D reconstruction methods noted above possess the “gap-filling”property to a greater or lesser extent. In particular, the mostresilient of these methods appear to be those based, not on classicalreconstruction methods, but rather on iterative, approximatingreconstruction methods that incorporate a mathematical model of asystem's geometrical and physical response to annihilation radiationfrom an imaging volume surrounded by depth-of-interaction capabledetector modules. The 3D OSEM algorithm with system modeling, forexample, is capable of making nearly distortion-free imagereconstructions including nearly perfect compensation for gaps accordingto the present invention.

As will be appreciated from the above, in accordance with the presentinvention, three-dimensional re-binning and/or reconstruction methodsare able to compensate for missing data introduced by axial gaps ordiscontinuities. Accordingly, a tomography scanner 10 may be designedsuch that an axial gap is purposefully incorporated into the scanner.Such a gap reduces the number of detector elements needed to span agiven axial field-of-view and permits light sources, such asphoswich/scintillator elements, to be optically coupled to sensors, suchas positron-sensitive photomultiplier tubes, with no light guides. Thepresent invention can be implemented with iterative, or statistical,reconstruction methods as well as with classical algorithms to providethe gap-filling function albeit with different amounts of axial imagedegradation.

Tomography scanners with axial gaps between detector rings in accordancewith the present invention can also include oscillatory or lineartranslation of the imaging bed or imaging gantry in the axial directionas shown in FIG. 3. For a scanner with an axial gap, the imaging bed orthe detector gantry can be driven to and fro in the axial directionduring imaging or translated uni-directionally, and transaxiallines-of-response acquired for parts of the object that would otherwisealways remain in the gap region. A translation of the bed or the gantrycan move a point in the object out of the gap region and into the regiondirectly sampled by the detector arrays. If the animal is translatedthrough the imaging field or the bed or gantry oscillates axially at areasonable frequency without moving the subject appreciably, anadditional benefit accrues, namely that the usual triangular-likesensitivity profile along the scanner's axis will appear to becomeflatter and the statistical properties of the acquired data will be moreuniform over the axial imaging field-of-view. It is understood that suchmechanical movements of the bed or gantry require the position of thebed or gantry relative to the target to be known at all times so thatlines of response can be transformed into a coordinate system fixed inthe subject rather than the gantry before image reconstruction.

Tomography scanners with axial gaps between detector rings in accordancewith the present invention can also use axially tilted detector modules.Conventional human and animal tomography scanners are usually designedsuch that the transverse dimensions of the detector array are constantin the axial direction. With the detector arrays tilted in the mannershown in FIG. 4, the effective axial gap width can be reduced. Thismanner of covering the imaging volume results in a variable ringdiameter as one moves along the scanner axis and a slightly increasingtransaxial gap width between modules as one moves toward the axialcenter of the system from each end. The effect of the gap is “smearedout” over the circumference of the array so its effect on the acquireddata is reduced. Accordingly, axial resolution degradation will be lesswith detector geometry of FIG. 4, and the 3D re-binning/reconstructionmethods noted earlier, or other post processing methods are moreeffective in restoring axial resolution in the gap region.

Typically, a PET scanner according to the present invention will be astationary ring-type scanner with an aperture appropriate for smallanimals. Such a scanner typically includes a gantry containing thedetector rings and an aperture into which the animal on an imaging bedcan be accurately inserted into the imaging field of view. Such scannersmight contain several computers for data acquisition and for dataprocessing. A particular embodiment for a small animal PET scanner has acylindrical detector array diameter between 10 and 40 cm, an aperturewithin two detector rings between 7 and 30 cm, a useful transversefield-of-view between 5 and 25 cm, an axial field of view between 4 and20 cm and an axial gap between detector rings of between 2 and 20 mm. Ina preferred embodiment, the animal would be surrounded bydepth-of-interaction capable detector modules to increase spatialsampling within the imaging volume, each formed of arrays of smallcross-section, e.g. 1-2 mm square, optically isolated scintillationcrystals or detector elements, numbering on the order of 100s ofdetection elements per module and 10s of thousands of elements for anentire scanner. As is understood, positron emission tomography involvesthe sensing of signals corresponding to three dimensional points ofinteraction in each of a pair of the detector modules in response todetection of time coincident photons from positron annihilations in abody containing a positron emitting compound.

Inasmuch as the present invention is subject to various modificationsand changes in detail, it is intended that all subject matter discussedabove and shown in the accompanying drawings not be taken in a limitingsense.

1. A tomography scanner comprising a plurality of axially aligneddetector rings forming a cylindrical detector array around a body to beimaged with well defined gaps between said detector rings, said detectorrings producing signals in response to radiation emanating from the bodyin three dimensions; and means responsive to said signals to producetomographic image reconstruction throughout the axial field-of-viewincluding said well defined gaps.
 2. A tomography scanner as recited inclaim 1 wherein said image reconstruction means includes meansperforming an iterative statistical or a classical image reconstructionprocess.
 3. A tomography scanner as recited in claim 1 wherein saidimage reconstruction means uses 3D OSEM and 3D MAP algorithms.
 4. Atomography scanner as recited in claim 1 wherein said imagereconstruction means includes an iterative statistical reconstructionmethod that models some or all of the physics and geometry of thedetector array.
 5. A tomography scanner as recited in claim 1 whereinsaid image reconstruction means utilizes 3D Fourier re-binning followedby 2D filtered back projection.
 6. A tomography scanner as recited inclaim 1 wherein said image reconstruction means utilizes a 3D Fourierre-binning followed by a 2D iterative statistical reconstruction method.7. A tomography scanner as recited in claim 1 wherein said imagereconstruction means utilizes a 3D reprojection algorithm.
 8. Atomography scanner as recited in claim 1 wherein said detector ringsinclude a plurality of detector modules angled in an axial direction. 9.A tomography scanner as recited in claim 1 and further comprising meansto provide relative reciprocating or unidirectional axial movementbetween the body to be imaged and said detector rings to reduce theeffects of said gaps.
 10. A tomography scanner as recited in claim 1wherein said detector rings include a plurality of detector modulescarrying a plurality of detector elements, each detector element havingan axial dimension and said well defined gaps have an axial dimensiongreater than or equal to said axial dimensions of said detectorelements.
 11. A tomography scanner as recited in claim 10 wherein saidwell defined gaps each have an axial dimension at least three timesgreater than said axial dimension of said detector elements.
 12. Atomography scanner as recited in claim1 wherein said detector rings eachcarry a plurality of detector elements having points of axial resolutionwith the spacing between said points defining pitch and said gaps havean axial dimension greater than said pitch.
 13. A positron emissiontomography scanner comprising a pair of detector rings forming acylindrical detector array around a body to be imaged with a welldefined gap between said detector rings; a plurality of detector modulescarried by said detector rings producing signals corresponding to thethree dimensional points of interaction in each of a pair of saiddetector modules in response to detection of time coincident photonsfrom positron annihilations in a body containing a positron emittingcompound; and means responsive to said signals to produce tomographicimage reconstruction of the spatial distribution of the amount ofpositron emitting compound in the body along the axial field-of-view ofsaid cylindrical detector array.
 14. A positron emission tomographyscanner as recited in claim 13 wherein said signal responsive meansincludes an iterative process.
 15. A positron emission tomographyscanner as recited in claim 13 wherein said signal responsive meansprovides classical reconstruction.
 16. A positron emission tomographyscanner as recited in claim 13 wherein said detector modules are angledin an axial direction to reduce the target area resulting from said gap.17. A positron emission tomography scanner as recited in claim 13 andfurther comprising means to provide relative reciprocating axialmovement between the animal and said detector array.
 18. A tomographyscanner as recited in claim 13 wherein said detector modules eachcontain at least one detector element having an axial dimension and saidwell defined gap has an axial dimension greater than said axialdimensions of said detector elements.
 19. A tomography scanner asrecited in claim 18 wherein said well defined gap has an axial dimensionat least three times greater than said axial dimensions of said detectorelements.